Drying and separating process and plant

ABSTRACT

A process and plant for drying and then separating moist salts, such as magnesium sulfite, in which the moist magnesium sulfite is heated to a temperature greater than or equal to about 200° C. in a dryer and the passed through a solid materials lock into a separating plant. The dryer is heated by a circulating stream of exhaust vapors which is heated in a heat exchanger by exhaust gas generated in the separating plant so that the water of crystallization of the magnesium sulfite as well as residual humidity is eliminated during the drying process. Vapor-free acid anhydride, usually sulphur dioxide, is prepared in the separating plant from the dehydrated salt and a metal oxide, e.g., MgO, is recovered or otherwise used. The acid anhydride may be liquified and has a large degree of purity.

BACKGROUND OF THE INVENTION

The present invention relates to a process for drying and thenseparating moist salts, such as magnesium sulfite, coming from a wetsulphur dioxide (SO₂) absorption plant into MgO, SO₂ and water vapor.The moist magnesium sulfite is dried in a dryer and then passed througha solid materials lock to a separating plant which is separated from thedryer thereby. A gaseous component of SO₂ separated from the magnesiumsulfite in the separating plant is used to heat combustion air needed toheat the separating plant and another gaseous stream of exhaust gasescaping from the separating plant is used to heat the dryer.

The present invention also relates to an installation for drying andsubsequently separating moist salts such as magnesium sulfite comingfrom a wet SO₂ absorption plant.

In chemical reclamation plants (such as those described in AustrianPatent No. 347,235) and in wet desulfurization plants operating on thebasis of MgO, as well as in other salt reclamation plants, a salt suchas MgSO₃ or Mg(HSO₃)₂ develops in a moist or dissolved state. If thissalt is separated for reclamation of the components, acid vapors such asSO₂ -water vapor mixtures develop at high temperatures, and suchmixtures are highly corrosive during cooling (condensation of the watervapor) and also operate to thin the acid to be obtained or cause theacid to be combined with the corrosion products. This hinders heatrecovery, equipment costs increase and productive power drops. Moreover,the corrosive vapors adversely affect the equipment, so that maintenanceexpenses or replacement expenses further lower the productive power ofthe chemical reclamation plant. Thus, the overall presence of SO₂ -watervapor mixtures is very problematic in salt reclamation operations.

Another reference, German Patent Publication No. DE 1 199 74 describes asimilar process for obtaining sulphur dioxide by using zinc oxidewhereby the main problem is to avoid sulphatizing the zinc in theseparation reactor. This problem does not occur in the present inventionwherein the main difficulty sought to be overcome is the corrosion ofthe elements of the separation plant due to the presence of sulphurdioxide-water vapor mixtures and the high heat requirements of theplant.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a newand improved drying and separating process and installation for moistsalts, such as magnesium sulfite, in which the problems described aboveare avoided.

It is another object of the present invention to provide a new andimproved drying and separating process and installation whichsubstantially eliminates the disadvantages mentioned above by separatingsulphur dioxide (SO₂) and steam (H₂ O) to prevent the formation ofsulphur dioxide-water vapor mixtures. As such, a pure (dry) acidanhydride (SO₂) is formed. In accordance with the invention, it has beenfound that the components affected by the pure (dry) acid anhydride(SO₂) are less subject to corrosion than if contacted with SO₂ -watervapor mixtures, so that the productive power of the plant isunexpectedly high.

In the process in accordance with the invention, the dryer in which themoist salts (MgSO₃) are situated is heated to a temperature of about200° C. and preferably more than 200° C. by the circulation of a portionof an exhaust vapor stream of the dryer through a system of conduits.The portion of the exhaust vapor stream of the dryer is heated as aresult of heat exchange with an exhaust gas being discharged from theseparation plant and directed through a system of conduits, i.e., bypassing the streams through separated tubes of a heat exchanger. Bymeans of the heating to a temperatures greater than about 200° C., inaddition to the moisture adhering to the salt, the water ofcrystallization of the salt in particular, is completely eliminated inthis drying operation. Thus, in the separation plant in accordance withthe invention, and in which the process in accordance with the inventionis applied, SO₂ with a purity value of about 70%, and preferably morethan 70%, is produced and MgO in the form of dust is produced (in theseparating plant) for feedback into an SO₂ absorption plant.

In the installation in accordance with the invention, the separatingplant is made in form of an indirectly heated rotary tubular kiln andthe dryer is made in form of a directly heated rotary tubular kiln or inform of a fluidized bed dryer, in particular with heating surfacesdipping into the fluidized bed. A feedback channel or other conduit isconnected thermically to the separating plant, i.e., via a heatexchanger, for part of the stream of the water/steam mixture produced inthe dryer. Preferably, the drying and separating plant is incorporatedthermically and in manufacture into an SO₂ absorption plant, inparticular for the reclamation of the boiling acid of a celluloseproduction. The dryer and the separating plant may also be incorporatedinto a SO₂ absorption plant of a caloric power plant, the separated SO₂being obtained in a liquefied form.

The installation may include a dust collection apparatus through whichthe partial exhaust vapor stream from the dryer is directed prior to itspassage through the heat exchanger. This dust collection apparatus has adust outlet side at which dust collects and an exhaust vapor side. Thedust collected at the dust outlet side is directed into the dryer. Aportion of exhaust vapor from the exhaust vapor side is passed to anexhaust vapor utilization system and another portion of exhaust vapor isdirected to the heat exchanger and therefrom into the dryer. Theseparating plant has an exhaust gas side which is connected to the heatexchanger.

As to the particulars of the SO₂ absorption plant, such a plant may havea liquid circuit and sludge side at which magnesium sulfite isaccumulated and which is connected to the dryer. The separating planthas a solid material side at which magnesium oxide is accumulated andwhich is connected to a liquid circuit of the SO₂ absorption plant.Also, the SO₂ absorption plant may include a compressor and a steamproducing unit having a steam/air pre-heater. A portion of the exhaustvapor stream from the dryer which is not passed through the heatexchanger is directed through this compressor to the steam/airpre-warmer. Moreover, the SO₂ absorption plant may comprise a steamproducing unit having a combustion chamber whereby the exhaust streamfrom the separating plant is incorporated into the combustion chamber inthe form of an oxygen carrier before the SO₂ absorption plant.

In addition, the invention makes it possible to effect a nearlyresidue-free desulfurization of caloric power plant exhaust gases inthat the metal oxide (MgO) is reused for absorption and develops in formof SO₂ and possible CO₂ in a salable state. The invention also makes itpossible to use dolomite, essentially CaMg(CO₃)₂, as the absorptionmeans for the economic production of pure MgO. In this case, of coursethe pure calcium sulfite which develops can be utilized or deposited,and the acid anhydride would be CO₂, with the advantage that through theobtaining of the MgO share, the entire calcium sulfite quantity isreduced to approximately 50% of what it was in SO₂ absorption plantsoperating on calcium base, so that the capacity of the gypsum market isnot exceeded.

Another application of the invention is to obtain pure CO₂ fromcombustion exhaust gases on basis of calcium, in which the burning ofthe calcium takes place outside the combustion and the CaO or Ca(OH₂) isused for the absorption of the CO₂ -containing, practically SO₂ -freeexhaust gas. The developing calcium slurry (CaCO₃) is then dried and thewater of crystallization is driven out for the major part, whereupon thewater-free CaCO₃ is separated thermically and the calcium oxide is againfed to the absorption process.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are illustrative of a preferred embodiment of the inventionand are not meant to limit the scope of the invention as encompassed bythe claims. The lines in the drawing connecting elements representconduits.

FIG. 1 is an illustration of a first embodiment of the invention.

FIG. 2 is an illustration of a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the accompanying drawings wherein like reference numeralsrefer to the same or similar elements, as shown in FIG. 1, in accordancewith the invention, a salt produced in a salt production plant isintroduced into a dryer 2 in a moist state, i.e., with adhering waterand with the crystal structure of incorporated water (water ofcrystallization), whereby the water contents are transferred completelyto the heating medium as a result of the heating in the dryer to atemperature greater than or equal to about 200° C. The water ofcrystallization is necessary to maintain the crystalline properties ofthe salt but is capable of being removed therefrom by sufficient heatwhich is present in the dryer 2. In the illustrated embodiment, the saltis MgSO₃.XH₂ O and the addition of H₂ O reflects the moist condition ofits entry into dryer 2.

When the dryer 2 is in the form of a rotary tubular kiln, a significantquantity of dust is carried into the dryer 2 with the heating medium(exhaust vapor as explained below), i.e., from the outlet end of thedryer 2 as shown in the drawing with respect to the inlet of the salt.The dust is removed from the dryer into a conduit, i.e., from the inletend of the dryer 2, and is returned to the dryer 2 after passing througha dust collection apparatus 4, e.g., a cyclone, having been dischargedfrom a dust outlet side thereof. After the dust collection apparatus 4,the resultant quantity of dust is returned to the dryer 2 with the salt.The exhaust vapor is thus removed from the dryer 2, and directed throughthe duct collection apparatus 4 to remove dust therefrom, and then aportion thereof is directed back into the outlet end of the dryer 2after it has been heated in a heat exchanger 5.

The dried salt produced in the dryer 2, from which the water ofcrystallization has been removed as well as the moisture adheringthereto, is directed through a solid materials lock 3 to a separatingplant 1 in which the acid anhydride is separated from the metal oxide.In the illustrated embodiment, the acid anhydride is SO₂ and the metaloxide is MgO. The gas/solid material separation is relatively simple.The separating plant is heated indirectly, and to a temperaturepreferably in the vicinity of about 700° C., so that the heating gasdoes not come into contact with the medium, the dried salt, to beseparated. The result of this indirect heating of the medium is that apractically pure acid anhydride, which in most cases is SO₂ or CO₂,accumulates and is unable to change into acid (with associated water)because of the absence of water and/or steam. In this manner,practically no dew point problem can occur in the separating plant 1 andthe elements of the plant are correspondingly protected from corrosion,corrosion which would otherwise be present due to the acids (with water)components.

Hot exhaust gas is directed through tubing and conduits from theseparating plant 1 in a circuit. A portion of the exhaust gas is heatedin a combustion plant 12, after it has passed through at least heatexchanger 5 and lost some of its heat, so that upon its departure fromthe combustion plant 12 and entry into the separating plant 1, thetemperature of the exhaust gas is slightly above the separatingtemperature, i.e., the temperature at which the separating plant isoperating. The recirculation of exhaust gas leads to a reduction ofNO_(x) contents therein. The separating plant 1 exhaust gas is firstcooled in the heat exchanger 5, wherein its heat is transferred to theexhaust vapor for the dryer 2, and thereafter the separating plantexhaust gas flows into an air heater 13. In air heater 13, fresh air isintroduced and is preheated with the intent to deliver the heated air tothe combustion plant 12 and, if necessary, to a steam producing unit 9.This stream of fresh air is further heated in a second air heater 14,prior to the combustion unit 12 and possible steam producing unit 9. Inthe second air heater 14, the acid anhydride coming out of theseparating unit 1, mainly SO₂, transfers its heat to this stream offresh air and is thus cooled down from the separating temperature.

The cooled separating plant exhaust gas, after it has passed throughheat exchanger 5 and air heater 13, contains a relative large amount ofheat and can therefore be used as a pre-heated oxygen carrier, whenadded to a quantity of oxygen, in the steam producing unit 9, e.g., of alye burning plant 16 in order to increase the heat turnover. The steamproducing unit 9 is heated by fuel 10 introduced therein which may bespent lye. Air from the second air heater 14 is further heated in asteam/air pre-heater 8, whereby the steam in the pre-heater 8 is theexcess exhaust vapor from the dryer 2 (that portion which did notrecirculate through the heat exchanger 5 and the dryer 2) which iscompressed in a compressor 7 prior to its passage through the pre-heater8. Thus, part of the combustion air being directed into and through thepre-heater 8 can be fed via the air pre-heaters 13 and 14.

The steam producing unit 9 can be an oil-fired power plant boiler theexhaust gas of which contains SO₂. In the plant boiler, SO₂ is absorbedby a metal oxide foam, e.g., MgO, with a view toward forming the salt orsulphate MgSO₃, whereby the salt MgSO₃ accumulates in the form of aninsoluble bottom sediment. This MgSO₃ -containing sediment may then beintroduced in form of a water/salt mixture into the dryer 2, preferablyafter at least an initial drying process. The use of a power plantboiler offers the advantage that the absorption medium is produced inthe plant itself and that the contaminant SO₂ occurs in a practicallypure and highly concentrated form, i.e., is salable. The economicsignificance now lies in an exhaust gas desulfurization plant which doesnot require any waste dump.

If the steam producing unit 9 is used in a cellulose plant on amagnesium bisulfite basis, as indicated by reference number 16, thespent cellulose lye is used as the fuel 10 and the exhaust gas containsa relatively large amount of SO₂, so that a bisulfite production plant11 can be intercalated or interposed between an SO₂ absorption plant 6and the steam producing unit 9, through which the magnesium bisulfitewhich is needed in the cellulose process is produced. The remaining SO₂can then be absorbed or recovered from the exhaust gas in the mannermentioned above, or can be made available for the cellulose process. Theplant furthermore makes it possible to obtain CO₂ from the SO₂ -freeexhaust gases of power plant boiler installations, whereby calcium orCaO is used or recycled as the absorption medium, for example. Theexhaust gas coming from the separating plant 1 which has given up partof its heat to the heat exchanger 5 or to the air heater 13 can now bemixed in the combustion chamber or into the exhaust gas before the flue,depending on the remaining heat contained in the steam producing unit 9,in particular in case of increased oxygen 5 contents, or also before thesulfite production unit 15, if the quantity of SO₂ is high.

Briefly, an SO₂ absorption plant 6 is an installation in which the SO₂content of a fluegas is absorbed by an absorbent, in this case eitherCaO or MgO. The flue gas comes from a steam generator 9 fired by coal oroil or another sulfur-containing fuel, preferably by "used boiling acid"of a cellulose production process containing lignin and other organicsubstances. The steam generator 9 is part of a caloric power plant or,in the case of a cellulose production operation, [art of the recoveryplant for the new boiling acid. At present, in view of environmentalconcerns, the fluegas must not contain SO₂ impurities. As such, an SO₂absorption plant is required. The absorption plant delivers MgSO₃ orCaSO₃ which must be split into MgO or CaO and SO₂, the MgO and CaO beingreusable in the absorption plant and the SO₂ salable in liquid form.

The installation can furthermore be also used for pure MgO reclamationfrom Dolomite, with calcium sulfite being produced as a waste materialif it cannot be used for gypsum production. Of course, only part of theSO₂ can then be reclaimed from the exhaust gas of the steam producingunit 9.

As shown in FIG. 2, instead of a kiln dryer 2 and associated dustcollection apparatus 4, a fluidized bed dryer 17 may be utilized. Afluidized bed dryer 17 includes heating surfaces and in a similar mannerto the embodiment described in FIG. 1, a moist salt is introducedtherein. Exhaust vapor is directed from the fluidized bed dryer 2 to azylon separator 18, the gas therefrom is removed as exhaust (a portionof which is later reheated and redirected through the fluidized beddryer 2) while the solid material is passed via the solid materials lock3 to the separating plant 1.

The examples provided above are not meant to be exclusive. Many othervariations of the present invention would be obvious to those skilled inthe art, and are contemplated to be within the scope of the appendedclaims. It is thus understood that a large variety of salts can be driedand separated in the inventive process and installation.

We claim:
 1. Process for forming dried sulfur dioxide from moistmagnesium sulfite, comprising the steps of:directing a by-productcomprising moist magnesium sulfite from a flue gas desulfurization tankinto a dryer; drying the moist magnesium sulfite in the dryer togenerate an exhaust vapor stream and dried magnesium sulfite; passingthe dried magnesium sulfite through a solid materials lock to a crackingplant separated by the solid materials lock from the dryer; cracking thedried magnesium sulfite in the cracking plant to form magnesium oxide,sulfur dioxide and an exhaust gas stream; directing a portion of theexhaust vapor stream from the dryer and the exhaust gas stream removedfrom the cracking plant into a heat exchanger to exchange heat; heatingthe dryer to a temperature of about 200° C. by circulating therethroughat least a portion of the exhaust vapor stream after it has been heatedin the heat exchanger such that moisture adhering to the magnesiumsulfite and water of crystallization of the magnesium sulfite issubstantially eliminated in the dryer; and removing dried sulfur dioxidefrom the cracking plant.
 2. The process of claim 1, further comprisingthe steps of: producing SO₂ in the cracking plant having a purity ofover about 70%; producing MgO in form of dust in the cracking plant; anddirecting the MgO into the SO₂ absorption plant.
 3. The process of claim1, wherein the dryer is heated to a temperature over 200° C.
 4. Aninstallation for formine: dried sulfur dioxide from moist magnesiumsulfite comprising:a dryer for receiving the moist magnesium sulfite andfor drying the moist magnesium sulfite to generate an exhaust vaporstream and dried magnesium sulfite; a cracking plant for receiving thedried magnesium sulfite and cracking the dried magnesium sulfite intomagnesium oxide, sulfur dioxide and steam; materials lock forcontrolling the passage of the dried magnesium sulfite from the dryerinto the cracking plant, said cracking plant being separated by thematerials lock from the dryer; means for directing a portion of anexhaust vapor stream from the dryer into said heat exchanger; means fordirecting a portion of the exhaust gas from the cracking plant into saidheat exchanger to exchange heat with said portion of said exhaust vaporstream; means for heating the dryer to a temperature over about 200° C.,said heating means comprising the circulation therethrough of saidportion of said exhaust vapor stream after it has been heated in saidheat exchanger, whereby moisture adhering to the magnesium sulfite andwater of crystallization of the magnesium sulfite is substantiallyeliminated in the dryer.
 5. The installation of claim 4, wherein saidcracking plant comprises an indirectly heated rotary tubular kiln andsaid dryer comprises a directly heated rotary tubular kiln.
 6. Theinstallation of claim 4, wherein dryer comprises a fluidized bed dryerhaving heating surfaces dipping into a fluid bed therein.
 7. An SO₂absorption plant including the installation of claim 4, wherein thedryer and the cracking plant are incorporated thermically and bymanufacture.
 8. The SO₂ absorption plant of claim 7, wherein the SO₂absorption plant has a liquid circuit and sludge side including the fluegas sulfurization tank in which magnesium sulfite is accumulated, saidsludge side of said absorption plant being connected to the dryer, andthe cracking plant having a solid material side at which magnesium oxideis accumulated, said solid material side of the cracking plant beingconnected to a liquid circuit of the SO₂ absorption plant.
 9. The SO₂absorption plant of claim 7, further comprising a compressor and a steamproducing unit having a steam/air pre-heater, a portion of the exhaustvapor stream from the dryer which is not passed through said heatexchanger being directed through said compressor to said steam/airpre-warmer.
 10. The SO₂ absorption plant of claim 9, further comprisinga steam producing unit having a combustion chamber, the exhaust streamfrom the cracking plant being incorporated into said combustion chamberin the form of an oxygen carrier before said SO₂ absorption plant. 11.An SO₂ absorption plant for the reclamation of boiling acid of acellulose production plant including the installation of claim 4,wherein the dryer and the cracking plant are incorporated thermicallyand by manufacture.
 12. An SO₂ absorption plant of a caloric power plantincluding the installation of claim 4, wherein, the separated SO₂ isobtained in a liquefied form.
 13. The SO₂ absorption plant of claim 12,wherein the SO₂ absorption plant has a liquid circuit and sludge sideincluding the flue gas sulfurization tank in which magnesium sulfite isaccumulated, said sludge side of said absorption plant being connectedto the dryer, and the cracking plant having a solid material side atwhich magnesium oxide is accumulated, said solid material side of thecracking plant being connected to a liquid circuit of the SO₂ absorptionplant.
 14. The installation of claim 4, further comprisinga dustcollection apparatus through which the partial exhaust vapor stream fromthe dryer is directed prior to its passage through the heat exchanger,said dust collection apparatus having a dust outlet side at which dustcollects and an exhaust vapor side, the dust collected at the dustoutlet side being directed into the dryer, a portion of exhaust vaporfrom the exhaust vapor side being passed to an exhaust vapor utilizationsystem and another portion of exhaust vapor being directed to said heatexchanger and therefrom into the dryer.
 15. The installation of claim 4,wherein the cracking plant has an exhaust gas side, said exhaust gasside of the cracking plant being connected to said heat exchanger.